All present varactor diode television tuners in the United States incorporate varactor diodes of the so-called hyperabrupt junction type. These diodes are characterized by a large capacitance ratio, that is, the ratio of the minimum to maximum capacitance exhibited by the diode under varying bias voltage conditions. One of the major considerations in using varactor diodes in tuners is that of getting the diode capacitance-changes-with-tuning-voltage-changes to track in the various tuner stages. Presently, tuner diodes are sorted a number of times to obtain a diode kit (group of selected diodes of corresponding characteristics) for use in a single tuner. Failure of any diode generally means that all diodes in the tuner are replaced with those in another kit. As may be expected, the sorting operations limit the diode "yield" and impose a substantial cost burden on these diodes. Additionally, there are other ancillary costs associated with manufacturing and servicing because of the need to maintain the integrity of the diode kits, both in the factory and in the service shop.
Even with hyperabrupt junction diodes, it is not feasible to cover the entire United States VHF tuning frequency range without employing some form of bandswitching. Bandswitching is generally accomplished by providing for a high and low inductance value in each tunable stage and switching to obtain a different inductance value to enable a given range of varactor diode tuning capacitance to tune over a different frequency range. Most varactor diode tuners incorporate mechanical switches for bandswitching. A very attractive automatic system is disclosed in U.S. Pat. No. 3,646,450 issued Feb. 29, 1972 to John Ma, assigned to Zenith Radio Corporation. In that patent a plurality of steering diodes selectively short out inductive elements upon occurence of a preselected tuning voltage (corresponding to the frequency gap between channels 6 and 7).
The most attractive advantage of abrupt junction varactor diodes is that their capacitance ratios, i.e., capacitance change vs. tuning voltage characteristic are predictable. These diodes may be incorporated in television tuners without being subjected to elaborate sorting and, as fully described in the above-entitled applications, it is sufficient to match their minimum and maximum capacitances. Indeed, with the highly stable precision resistor divider assembly disclosed and claimed in application Ser. No. 502,482, it is feasible to specify only one end point for the abrupt junction diodes.
In varactor diode tuners, the effective tuning voltage range that may be employed is limited at one end by the diode voltage breakdown characteristic and, at the other end, by the tuner signal handling capability. The limited capacitance change available with abrupt junction diodes may present some difficulties in strong signal areas at the lower VHF channel tuning voltages. For example, the tuning voltage for channel 2 may be on the order of -0.9 volts DC and the forward conduction voltage for the silicon type varactor diode is +0.7 volts DC. Consequently, if the peak-to-peak signal voltage exceeds 3.2 volts across the diode, the varactor diode will be driven conductive on peak positive signal excursions. In certain signal locations this may not be an uncommon occurrence.
Practically, the receiver automatic gain control system usually provides for very large signal gain reductions for the signal in the intermediate frequency stages of the receiver and in the interstage coupling of the tuner. Thus it is the antenna or RF input stage of the tuner which is subject to the signal overload condition with which this invention is concerned.
When the nature of the television signal is considered, it will be seen that the peak portions represent synchronizing pulse information. An overload condition can thus distort or destroy the synchronizing pulses in the signal, rendering the information unrecoverable for viewing.